The present invention generally relates to processes for producing components having regions with different microstructures. More particularly, this invention is directed to a technique for producing components, as an example, rotating components of a turbomachine, by performing an incremental forging process on a preform to yield different microstructures within regions of the resulting component.
Components within the combustor and turbine sections of a gas turbine engine are often formed of superalloy materials in order to achieve acceptable mechanical properties while at elevated temperatures resulting from the hot combustion gases produced in the combustor. Higher compressor exit temperatures in modern high pressure ratio gas turbine engines can also necessitate the use of high performance superalloys for compressor components, including blades, spools, disks (wheels) and other components. Suitable alloy compositions and microstructures for a given component are dependent on the particular temperatures, stresses, and other conditions to which the component is subjected. For example, rotating hardware such as turbine disks and compressor spools and disks are typically formed of alloys that must undergo carefully controlled forging, heat treatments, and surface treatments to produce a controlled grain structure and desirable mechanical properties. Notable examples of alloys used in these applications include gamma prime (γ′) precipitation-strengthened nickel-base superalloys containing chromium, tungsten, molybdenum, rhenium and/or cobalt as principal elements that combine with nickel to form the gamma (γ) matrix, and contain aluminum, titanium, tantalum, niobium, and/or vanadium as principal elements that combine with nickel to form the gamma prime precipitate strengthening phase, principally Ni3(Al,Ti). Particular examples of gamma prime nickel-base superalloys include René 88DT (R88DT; U.S. Pat. No. 4,957,567), René 95 (R95; U.S. Pat. No. 3,576,681), and René 104 (R104; U.S. Pat. No. 6,521,175), as well as certain nickel-base superalloys commercially available under the trademarks Inconel®, Nimonic®, and Udimet®. Disks and other critical gas turbine engine components are often forged from billets produced by powder metallurgy (P/M), conventional cast and wrought processing, and spraycast or nucleated casting forming techniques. Forging is typically performed on billets have a fine-grained microstructure that promotes formability, after which a heat treatment is often performed to cause uniform grain growth (coarsening) to optimize properties. This heat treatment is performed at a supersolvus temperature, in other words, above the solvus temperature at which the gamma prime precipitates of the alloy enter into solid solution.
A turbine disk 10 of a type known in the art is represented in FIG. 1. The disk 10 generally includes an outer rim 12, a central hub or bore 14, and a web 16 between the rim 12 and bore 14. The rim 12 is configured for the attachment of turbine blades (not shown) in accordance with known practice. A bore hole 18 in the form of a through-hole is centrally located in the bore 14 for mounting the disk 10 on a shaft, and therefore the axis of the bore hole 18 coincides with the axis of rotation of the disk 10. The disk 10 is a unitary forging and representative of turbine disks used in aircraft engines, including but not limited to high-bypass gas turbine engines such as the GE900 and GEnx® commercial engines manufactured by the General Electric Company.
The bore 14 and web 16 of the turbine disk 10 (as well as those of compressor spools and disks) generally have lower operating temperatures than the rim 12. It is therefore permissible and often desirable that the bore 14 have different properties than the rim 12. Depending on the particular alloy or alloys used, optimal microstructures for the rim 12, bore 14 and web 16 can also differ. For example, a relatively fine grain size is often optimal for the bore 14 and web 16 to promote tensile strength, burst strength, and resistance to low cycle fatigue (LCF), while a coarser grain size is often optimal in the rim 12 to promote creep, stress-rupture, and crack growth resistance, for example, low dwell (hold-time) fatigue crack growth rates (DFCGR) at high temperatures. To satisfy these competing requirements, disks have been proposed that are formed of multiple alloys and/or have different microstructures within the rim and bore. For example, U.S. Pat. Nos. 4,820,358, 5,527,020, 5,527,402 and 6,478,896 disclose dual heat treatment techniques capable of producing single-piece, constant-composition disks having coarser grains within the rim and finer grains with the bore as a result of performing heat treatments at different temperatures on the rim and bore, thereby obtaining the different grain structures and resulting different properties.
Multiple alloy disks that have been investigated typically entail the fabrication of separate rim and bore portions formed of different alloys. The rim and bore portions are then joined together, such as by welding or another metallurgical joining process. One such example is known as forge-enhanced bonding which, as disclosed in U.S. Pat. Nos. 5,100,050, 5,106,012 and 5,161,950, entails simultaneously forging preforms of the rim and bore. During the forging operation, deformation of the preforms yields the rim and bore as well as results in metallurgical joining of the rim and bore. Another example is solid-state welding processes, which include inertia welding techniques of the types disclosed in U.S. Pat. No. 6,969,238 and U.S. Published Patent Application Nos. 2008/0120842 and 2008/0124210. Because the different alloys may have different solvus temperatures such that the alloys are not conducive to a common solution heat treatment cycle, inertia welding has been limited to joining solution heat treated rim and bore portions, which are then subjected to an aging cycle after the welding operation.
Forging temperatures, strain, and strain rates profiles and post-forging cooling rates have also been shown to influence grain sizes within single-piece, constant-composition disks formed of gamma prime nickel-base superalloys. For example, U.S. Pat. No. 5,593,519 discloses a forging technique capable of producing uniformly coarse grains by maintaining low strain rates (0.01 s−1 or less) when forging at a supersolvus temperature, generally up to about 100° F. (about 55° C.) above the gamma-prime solvus temperature. Different grain sizes can be obtained in specific locations of a component by cooling the specific locations at different rates from the supersolvus forging temperature. As another example, U.S. Pat. No. 6,059,904 discloses a forging technique capable of producing uniformly fine grains by maintaining low strain rates (0.01 s−1 or less) during at least a first forging step performed at a subsolvus temperature, generally as low as about 100° F. (about 55° C.) below the gamma-prime solvus temperature. This patent reports that different grain sizes can be obtained in specific locations of a component by utilizing the teachings of U.S. Pat. No. 5,593,519.
Even with the advancements outlined above, in practice current certified commercial flight turbine disks have only been produced as monolithic structures formed by a single alloy and processed to have a uniform microstructure whose grain size is necessarily a compromise between the creep, stress-rupture and DFCGR properties desired for the rim and the LCF and burst properties desired for the bore.